Following a failed attempt to invade the capital city of Umbara,[8] Krell and the 501st were dispatched on a new assignment to secure a heavily fortified Umbaran airbase that was critical to the Republic's strategy. At the General's command, CaptainCT-7567 ("Rex") led a vanguard assault on the base's defense perimeter while Appo and the majority of the 501st remained at Krell's side as he observed the fighting from a distance. After the Umbarans' defenses were sabotaged by two clone infiltrators, the ARC trooperCT-5555 ("Fives") and the trooper known as "Hardcase," Krell gave the order for a full-scale attack, allowing Appo to deploy the rest of the legion to occupy the airbase.[9]

In the aftermath of the battle for the airbase, Fives and another trooper, CT-5597 ("Jesse") were placed under military arrest for acts of insubordination against Krell.[10] Although they were originally scheduled to be court-martialed, the General used his authority to sentence them both to summary execution. Appo escorted the rogue clones to the site of their execution, although the firing squad ultimately refused to carry out the order. Shortly afterward, it was discovered that Krell had been secretly supporting the Separatist cause after falling to the dark side of the Force. In response to the fallen Jedi's acts of treason, Appo and the 501st Legion mutinied against Krell with the support of the 212th Attack Battalion. Despite sustaining a number of casualties, they successfully apprehended the former Jedi. Krell was ultimately executed by the trooper "Dogma," who was originally the former Jedi's most loyal follower until Krell admitted his betrayal.[2]

The attack on the Jedi headquarters drew the AlderaaniansenatorBail Organa to the Temple, only to be barred from entering the facility by Appo and a contingent of the 501st. The commander claimed that the Temple had become the site of a "Jedi rebellion" and threatened Organa at the point of a blaster when the politician initially ignored the clone's attempt to block his path. However, their encounter with Organa distracted the clones from the approaching Padawan Zett Jukassa, who managed to kill and wound several soldiers before he was fatally shot by a 501st trooper.[3]

For most of his life, Appo had been trained for service as a soldier of the Galactic Republic.[5] Like his clone brethren, he was also conditioned to be absolutely loyal to the chain of command, obeying every order without question.[13] Though he witnessed firsthand Pong Krell's willingness to sacrifice clone lives for victory,[1] Appo nonetheless remained loyal to his general until it was discovered that the Besalisk Jedi had betrayed the Republic.[2] As a result of his programming, in the form of a control chip in his brain,[12] Appo was forced to comply with Order 66.[3]

As a clone of Jango Fett, Appo was physically identical to his humanmaletemplate in height and appearance. He stood at 1.83 meters and inherited his progenitor's hair and skin pigmentation as well.[1]